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The number of people being treated for carbon monoxide poisoning has risen.
Dr John Duncan, director of naval medicine at the Royal New Zealand Navy base in Auckland, said the number of accidental carbon monoxide poisonings his staff treated at its hyperbaric chamber usually amounted to a handful of patients each year.
But numbers had risen recently with people using concrete cutters in enclosed spaces one of the main reasons for the increased admissions.
"When they're used indoors without ventilation or some form of protection they can be deadly," said Dr Duncan.
Derek Hamilton and Martin Janse Van Rensburg died of suspected carbon monoxide poisoning at a Waikato motor camp when they mistakenly took a portable barbecue into their cabin last Saturday.
Survivor Jayson Basson was treated at the Naval base's hyperbaric chamber before being moved to North Shore Hospital.
Dr Duncan said other admissions to the chamber included a "quite ill" person using a fuel-driven water blaster to clean the inside of a water tank and forklift operators driving inside poorly ventilated warehouses.
"It's particularly common," he said.
A colourless, odourless and tasteless gas, carbon monoxide can be produced by the incomplete burning of fuels or virtually any form of combustion.
It affects the blood's ability to carry oxygen and depending on the volume and level of exposure can kill within a minute.
Dr John Fountain, a medical toxicologist from the National Poisons Centre, said exposure symptoms included nausea, fatigue and breathing difficulties which can lead to headache, flushing and a rapid heartbeat.
"Obviously if you are exposed to high concentrations for a longer period of time you become quite confused and disoriented, lose consciousness, go into a coma and then die," said Dr Fountain.
The treatment involves oxygen therapy: high-flow oxygen through a mask at normal atmospheric pressure, or being put into a hyperbaric chamber at increased pressure.
He said the effects could sometimes not become apparent for a couple of weeks and there was a possibility of sufferers having ongoing neuropsychological effects.
Dr Fountain said appliances such as gas heaters, cookers and burners needed to be serviced regularly.
But Dr Duncan said the best form of protection was either "substitution or elimination of the hazard".
"Don't cook in enclosed spaces with gas cookers and if you need the power of a petrol motor you need to ensure there is adequate ventilation."
